The invention relates to electrically actuated aircraft brakes and is more particularly but not exclusively concerned with the wear measurement and adjustment of such brakes.
Aircraft brakes are typically of a multi disc type with carbon-carbon composite (C—C) friction discs and feature hydraulically actuated rams (pistons) to generate the clamping force across the brake heat pack to generate friction at the disc interfaces. A brake heat stack comprising C—C stator discs keyed to a non-rotating torque tube and C—C rotor discs keyed to the rotating wheel and interleaved between the stators generates the friction forces for braking and absorbs the kinetic energy of the aircraft as heat. Brake actuation is under the control of an electronic control unit (ECU) controlling braking force in response to a brake demand signal and monitoring braking through signals representing parameters such as brake pressure, brake torque, deceleration, wheel speed and skid activity.
It is important for aircraft safety that the brake heat stacks have sufficient capacity to absorb the kinetic energy of the aircraft during an emergency braking event such as a Rejected-Take-Off. This requirement dictates a minimum heat stack mass that must be available and it is critical that the amount of material remaining in the brake heat stack, normally identified by the thickness of the heat stack, can be monitored to ensure that heat stacks are replaced at the appropriate time.
Brake heat stack thickness is monitored manually by examining the length of a wear pin attached to the brake stator disc at the end of the brake heat stack where the brake pressure is applied. Such wear pins indicate the thickness of material remaining in the brake heat stack before maintenance action is required.
As technology is introduced for the more electric aircraft there is an emerging trend towards using electrically powered actuator rams for aircraft brakes. In such actuators the movement to apply and release the brake clamping force in the actuator ram is driven by an electric motor through a mechanism such as gears or ball screws. The use of electric actuation allows the actuator to become more intelligent with the capability to provide information such as actuator position to the brake control system.
U.S. Pat. No. 6,003,640 in the name of Goodrich describes a system using position sensors coupled to the actuator ram to determine actuator ram position. By detecting the position of the actuator when it is in contact with the closed brake heat stack during a calibration routine and comparing this position with a previously determined reference position the brake heat stack wear is determined.
The use of wear pins in hydraulically actuated brakes and, for electric actuators, a system of the type proposed in U.S. Pat. No. 6,003,640, takes no account of brake temperature when determining heat stack position and wear of the brake heat stack.
Expansion of the C—C brake heat stack is typically in the order of 12×10−6° C.−1. This is equivalent to 1.2 mm per 1000° C. per 100 mm of heat stack thickness. For a typical medium size civil aircraft carbon-carbon brake heat stack with total thickness of 200 mm this gives an expansion of 2.4 mm between ambient and 1000° C. For a typical civil aircraft carbon-carbon brake heat stack with total thickness of 300 mm this gives an expansion of 3.6 mm between ambient and 1000° C.
Thermal expansion of a brake friction material with a positive expansion coefficient will be a positive value when the brake heat stack is increasing in temperature and a negative value when the brake heat stack is cooling. When the brake heats during a braking cycle the heat stack will expand. When the brake cools down between braking cycles the heat stack will contract
If the brake heat stack thickness is measured when the brake heat stack is at an elevated temperature then, when the heat stack then cools below the temperature at which the heat stack thickness was measured, the brake heat stack will contract due to the cooling, thereby decreasing the heat stack thickness. If the system is monitoring brake wear or amount of wear remaining in the brake heat stack a distorted assessment of wear or remaining material will be obtained, resulting in an incorrect evaluation of remaining cycles to overhaul if the system uses algorithms to assess remaining brake life.
Brake control systems such as that featured in WO 02/12043 can now use information on heat stack thickness, derived from amount of material worn away from new or the amount of material remaining above heat pack fully worn thickness to determine the remaining life of the brake heat stack before removal is required.
To obtain a measurement of brake heat stack thickness that can be used to derive accurate information on brake wear the determination of heat stack thickness can only be carried out when the brake is at ambient temperature, or at a temperature at which expansion is considered to be negligible.
If automatic wear measurement is to replace visual inspection of the brakes, several problems must be overcome. Currently, it is the responsibility of the pilot to carry out a visual inspection of components such as the tyres, engines and brakes to ensure the aircraft meets operational requirements.
If wear pins are removed from brake assemblies the pilot requires an indication of the health of the brake so that the pilot can fulfil his responsibilities. A user initiated wear measurement would give the necessary information, however the system detailed in U.S. Pat. No. 6,003,640 can only operate when the brake is at ambient temperature. As styles of operation change, some airlines run aircraft on an almost back to back schedule of flights with several different pilots flying any individual plane in a given tour of duty. Each one of these pilots must obtain an accurate wear measurement and will not be able to rely on a previous measurement taken that day as it could have been taken 20 flight cycles earlier.
Airlines are under increasing pressure to fully utilise their asset, so pressure to reduce turn round times—particularly for shorter flights, is high. The limiting factor is often brake temperature, as the brake must be capable of a full Rejected Take Off (RTO) before the aircraft can be released. Typically the limiting temperature is set at around 300° C., though it can be as high as 400° C. This could relate to an axial expansion of over 1 mm on a 300 mm heat stack, resulting in an unacceptable error in wear measurement. For instance, if 0.1 mm of wearable material was remaining at the start of a tour of duty, heavy wear or contamination of the heat stack with de-icer could occur and cause the brake to go below safe limits for heat stack size. Any wear measurement taken without temperature compensation at a temperature of 50° above ambient could result in a brake giving an indication that it is in a healthy condition when it is in fact below the allowable limit.
For electrically actuated brakes the opportunity to carry out a calibrated heat stack wear measurement of the type proposed by Goodrich in U.S. Pat. No. 6,003,640 is limited to maintenance periods where the aircraft is out of service. In addition, information on heat pack wear is required to track wear of the heat pack in order to inform that maintenance is required so there is a need for representative wear information to be recorded on an ongoing basis during operating cycles. This cannot be provided if the actuator ram position for contact with the closed heat stack heat pack thickness is being determined as the position will change with brake temperature.